Toughness is the ability of a material to absorb energy and undergo large permanent set without rupture. For many engineering adhesive applications, toughness is often the deciding factor. Plastics, because of their inherent brittleness, have been modified in a variety of ways in efforts to improve the toughness thereof. Epoxy resins, for example, which form a versatile glassy network, exhibit excellent resistance to corrosion and solvents, good adhesion, reasonably high glass transition temperatures (Tg) and adequate electrical properties. Unfortunately, however, the poor fracture toughness of epoxy resins oftentimes limits the usefulness thereof.
The impact strength as well as most other physical properties of crosslinked epoxy resins is controlled by the chemical structure and ratio of the epoxy resin and hardener, by any added fillers, and by the curing conditions used. Unfortunately, crosslinked, glassy epoxy resins with relatively high Tg (>100° C.) are brittle. The poor impact strength of high glass transition epoxy resins limits the usage of epoxies as structural materials and in composites.
Indeed, current commercially available underfill epoxy adhesives are excessively brittle and tend to fail prematurely in such applications as chip scale packaging (CSP) and related applications as a result of poor fracture toughness. Conventional toughening agents (e.g. carboxyl terminated butadiene, i.e., CTBN) are frequently unsuitable as additives in these adhesives because they adversely affect the capillary flow properties of the uncured adhesive.
Accordingly, there is a need for toughening agents that are effective for improving the toughness of adhesive formulations, especially in formulations requiring good capillary flow properties.